Field of the Invention
This invention relates to a magnetic disc memory device which can enhance the recording density of magnetic discs.
Description of the Prior Art
In conventional magnetic disc memory devices, magnetic discs are mounted on a shaft member which in turn is supported by a housing through a ball bearing. The ball bearing is lubricated by a lubricant such as oil or grease, and a rotor mounted on the shaft member is opposed to a stator mounted on the housing. A case disposed over the shaft member is mounted on the housing and this case hermetically seals a rotatable member through or not through a filter which communicates the gas in the case with the atmosphere.
Now, the balls, the inner race track, the outer race track and the retainer of a ball bearing are not complete in sphericalness and circularity, and the ball bearing has a portion which moves while being in solid contact with rotation. Accordingly, in the ball bearing, irregular vibration occurs during rotation and this irregular vibration causes vibration of the shaft member and magnetic discs. Magnetic signals are recorded in the form of concentric circles on the magnetic discs, but since the magnetic discs vibrate, the intervals between the adjacent circularly recorded portions cannot be narrowed to enhance the recording density. Also, when the ball bearing rotates, dust, foreign materials and evaporating substances exhale from the lubricant of the ball bearing and adhere to the surface of the magnetic discs and the surface of a magnetic head opposed to the magnetic discs. Therefore, reading and writing of the magnetic discs becomes impossible, and the magnetic head contacts the magnetic discs with the dust interposed therebetween and the magnetic head is damaged thereby. To prevent contamination of the magnetic discs and magnetic head, a contact type seal, a non-contact type seal or a magnetic fluid seal is sometimes used for a ball bearing. However, these seal structures are not perfect inasmuch as the rotation torque becomes great or sealing is insufficient, and moreover are costly. Furthermore, the ball bearing is great in friction moment and therefore, the power of the motor becomes great and it is difficult to make the motor compact. Also, both of the ball bearing and the motor having a rotor and a stator as the driving mechanism generate heat and transfer of the heat is non-uniform and thus, the temperatures of the various portions of the magnetic discs become non-uniform. The various portions of the magnetic discs are non-uniform in thermal expansion and therefore, positional deviation occurs to the various portions of the magnetic discs and the recording density of the magnetic discs cannot be enhanced.